Storage tanks are widely used for the bulk storing of a great variety of liquids. Some of these liquids are highly corrosive and/or are flammable. There is a recognized need for such tanks to have a secondary containment means which is capable of containing leaked liquid from the storage tank. Rigid double walled tanks made of metal have been suggested as one alternative. While effective for containment purposes, such tanks, as presently available, are costly to build and difficult to install because of their weight. The tanks are built by basically forming two rigid tanks with different diameters and then placing one tank inside the other.
Double walled tanks made from fiberglass reinforced resinous material are also known. They are built using a number of distinct time consuming steps. In all known commercial methods, a cylindrical-shaped, reusable mold is used to build tank halves which are subsequently assembled to form a single wall tank. Next, a larger diameter reusable mold is used to build a horizontal half-tank. The fiberglass/resin is applied in a known manner to the mold and cured to form the half-tank. A second horizontal half-tank is formed. Next, the completed inner tank is placed into the larger diameter half-tank. The second larger diameter half-tank is placed over the inner tank, joined and sealed at the seams with its matching half-tank. The resultant product is a double walled storage tank system comprised of essentially two rigid tanks, one inside the other.
In recent years there have been suggested improved methods of building double walled storage tanks. U.S. Pat. Nos. 4,561,292, 4,640,439, 4,644,627, 4,744,137, 4,780,946 and 4,780,947 disclose double walled storage tank systems wherein means are used to maintain a spaced relationship between the walls of the tanks. The spaced relationship is needed for effective leak detection purposes. However, the spacing means suggested require a separate time consuming step in properly positioning some specially constructed material on an inner tank wall prior to forming an outer tank wall. U.S. Pat. Nos. 4,859,262 and 4,871,078 disclose double walled storage tank systems which also use special materials, though the systems are more efficiently built.
There has now been discovered methods whereby new and used storage tanks can be provided with secondary containment means in a convenient, yet economical manner. Further, used storage tanks are refurbished to a standard equivalent to that possessed by a new tank and then upgraded to have a secondary containment feature.